In order to reduce emissions of air pollutants, and especially nitrogen oxides (NOx), the United States Environmental Protection Agency (EPA) has issued strict emissions standards for the power generation industry, including gas, oil, and coal-fired power plants. NOx is generated from the combustion of any fuel in the presence of air. In typical gas turbine systems, the majority of NOx is generated by the pilot flame, which burns at a high temperature and stabilizes the combustion of a leaner supply of premixed fuel and air. Because NOx is an undesirable gas in the environment and its emission must be limited, the conventional wisdom is to produce as little NOx as possible in the combustion process, such as by delivering as little fuel to the pilot flame as is necessary to achieve flame stability at various power levels.
Nevertheless, some NOx is always produced and the amount of NOx produced increases as the power level of the turbine increases. Post-combustion treatment methods such as selective catalytic reduction (SCR) are known to reduce the amount of NOx in the exhaust gas stream to low levels prior to release to the environment. Various reducing agents can be used in SCR systems, including hydrogen, ammonia, urea, etc. Ammonia is one of the most efficient reducing agents for reducing NOx emissions to low levels. However, ammonia is also harmful to the environment, and ammonia slip can occur when ammonia passes through the SCR system into the environment without reacting with NOx. U.S. Pat. No. 7,166,262 describes a method for minimizing ammonia slip by closely regulating the flow of ammonia in response to detected ammonia and nitrogen oxide levels. However, even with the tight regulation of ammonia levels, the amount of ammonia can fluctuate substantially during load changes of the corresponding engine.